1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device. More particularly, the present invention relates to a method for forming a pullback opening above a shallow trench isolation (STI) structure.
2. Description of the Related Art
Due to the rapid development of integrated circuit manufacturing techniques, highly miniaturized and integrated devices are now fabricated. As dimensions of each device shrink, isolating structures between device have to shrink correspondingly. Hence, the process of forming device isolation becomes harder. Conventionally, devices are isolated by forming a field oxide layer by a local oxidation (LOCOS) method. However, the field oxide layer is subject to bird's beak encroachment, which is a considerable barrier to device miniaturization.
In the meantime, other types of device isolation structures have also been developed. One widely adopted device isolation method, especially in the fabrication of sub-half micron integrated circuits, is shallow trench isolation (STI).
To form a conventional STI structure, a pad oxide layer and a silicon nitride mask layer are formed in sequence over a substrate. A photolithographic process is next performed to pattern out a trench region. Using a dry etching method, the silicon nitride layer, the pad oxide layer and the substrate are sequentially etched to form a trench in the substrate. A region surrounded by the trench becomes an active region where active devices are subsequently formed. Thermal oxidation is carried out to form a liner oxide layer over the interior surface of the trench. Silicon oxide is deposited into the trench and over the silicon nitride layer by chemical vapor deposition. Silicon oxide that rises above the silicon nitride layer is removed by chemical-mechanical polishing to form an isolating structure in the trench. Finally, the silicon nitride layer is removed using hot phosphoric acid solution and the pad oxide layer is removed using hydrofluoric acid solution.
In the fabrication of deep submicron devices, available space between neighboring active regions is very small. Hence, only narrow trenches can be formed. When a trench is very narrow, the gap-filling capability of the trench with respect to the deposition of silicon oxide deteriorates. Consequently, structural defects such as voids or seams are more likely to form inside the silicon oxide plug of an STI structure. Therefore, in the latest development, a `pullback` process for widening the opening leading to the STI trench is introduced to facilitate the deposition of silicon oxide.
FIGS. 1A and 1B are schematic cross-sectional views showing a first method of manufacturing a conventional STI structure with a pullback opening. As shown in FIG. 1A, a pad oxide layer 110 and a silicon nitride layer 120 are formed in sequence over a substrate 100. Using photolithographic and etching processes, a trench 140 having an opening width of m is formed in the substrate 100. As shown in FIG. 1B, a pullback process is next carried out to widen the opening at the top of the trench 140 by performing an isotropic etching operation using hot phosphoric acid. After the etching step, the silicon nitride layer 120 surrounding the trench opening is pulled back a distance of about n, roughly equivalent to about 200 .ANG.. Therefore, the width of the opening above the trench 140 is increased to m+2n.
FIGS. 2A and 2B are schematic cross-sectional views showing a second method of manufacturing a conventional STI structure with a pullback opening. As shown in FIG. 2A, a pad oxide layer 110, a silicon nitride layer 120 and a silicon oxide layer 130 are formed in sequence over a substrate 100. Using photolithographic and etching processes, a trench 140 having an opening width of m is formed in the substrate 100. As shown in FIG. 2B, a pullback process involving the etching of both the silicon nitride layer 120 and the silicon oxide layer 130 is next carried out to widen the opening at the top of the trench 140. The silicon nitride layer 120 and the silicon oxide layer 130 are simultaneously etched using an isotropic etching agent such as hydrofluoric acid in a glycerol or ethylene glycol (EG) solution. Ultimately, the silicon nitride layer 120 and the silicon oxide layer 130 surrounding the trench opening are pulled back a distance n, roughly equivalent to about 200 .ANG.. Similarly, the width of the opening above the trench 140 is increased to m+2n.
Both the first and the second pullback processes depend on wet etching. However, the pullback distance n is rather difficult to control in wet etching operations because the first section of the silicon wafer clipped into the acid solution is usually the last section pulled out of the acid solution. Consequently, different areas of the wafer remain in the acid solution for different periods of time, causing the pullback distance to vary substantially across the wafer.